1. Field of the Invention
The present invention relates to the field of speed detecting devices employing a Doppler radar.
2. Description of the Prior Art
Generally, when a vehicular speed is detected using a Doppler radar, the amplitude of the Doppler signal changes due to fading of radar waves, resulting in errors in the speed detection.
Particularly, when the speed of low-speed vehicles, such as construction vehicles, is detected, the effects of vehicular speed errors are serious because frequency of Doppler signal is low, and repetition time of the fading is long.
In order to eliminate such effects of vehicular speed detection errors at the time of low vehicle speed, speed detection devices which detect fading and reduce vehicle speed detection errors based on the size of the fading have been typically proposed. Such devices work effectively when the fading depth is large, but they are unable to prevent speed detection errors from occurring satisfactorily when the fading depth is small, since fading cannot be detected in such case.